The present invention relates to bearings, materials for bearings and to a method for the production thereof.
Bearings used in modern engines need to possess a combination of often conflicting properties. Bearings generally comprise several layers (see FIG. 2): a backing layer 2 of a strong material such as steel or bronze having a thickness in the range from about 1 to about 10 mm; a layer of a bearing alloy 4 usually based upon alloys of copper or aluminium and having a thickness in the range from about 0.1 to about 1 mm; and, optionally a so-called overlay layer 6 on top of the bearing alloy layer and having a thickness in the range from about 5 to about 50 μm. There may also be additional layers: one situated between the backing 2 and the bearing alloy layer 4 to assist in enhancing adhesion between these two layers and comprising, for example, a thin layer (generally about 5 to 50 μm, although much thicker layers up to about 300μ have been known) of aluminium or an aluminium alloy, nickel or another material as are known in the bearing art in the case of an aluminium-based bearing alloy 4. A further layer, a so-called interlayer, may be situated between the bearing alloy layer 4 and the overlay layer 6 and be present for the purpose of enhancing adhesion between the two layers and/or acting as a barrier to prevent or minimise unwanted diffusion of alloying constituents between the two layers. Such interlayers are usually very thin, of the order of about 0.5 to 5 μm.
Where present, the overlay layer provides the actual running or sliding surface between the bearing itself and a co-operating shaft journal. The overlay is generally a relatively soft material being based upon alloys having tin, lead, cadmium or aluminium as their main constituent. The purpose of the overlay, which is generally softer than the bearing alloy layer, is to provide a conformable layer able to accommodate small misalignments between the bearing and shaft journal caused due to imperfections in the machining processes involved in the bearing and engine manufacturing processes, i.e. the overlay possesses the characteristic of conformability. The overlay layer must also be seizure resistant, fatigue resistant, corrosion resistant, wear resistant and provide for embeddability of dirt and debris carried in the lubricating oil. Good fatigue resistance and wear resistance are generally associated with high strength and hardness. Good seizure resistance requires the material forming the running surface to have good compatibility which overlay alloys, due to their composition, generally possess. Similar requirements are also associated with the bearing alloy layer where no overlay is present and the bearing alloy itself forms the actual running or sliding surface. However, it should be born in mind that in some engines, due to the arduous service conditions, it is common for the overlay layer to be worn away thereby exposing the underlying bearing alloy layer which then becomes the actual sliding or running surface.
However, whilst wear resistance of such soft metal overlay layers is poor with modern, highly rated engines, the fatigue resistance is often better than might be expected having regard to the properties of the bulk alloys. This is due to the conformability of such alloys spreading the applied load over a greater area and ameliorating the effects of point loading on the underlying bearing alloy substrate layer which would otherwise occur without the soft layer.
Overlay materials based on alloys of tin or lead or cadmium have generally been deposited by electroplating techniques from aqueous solutions. Attempts to improve the strength and wear resistance of such alloys have led to the development of alloys comprising lead-tin-copper and to similar alloys but containing a proportion of hard particles co-deposited with the alloy and distributed throughout the alloy matrix. Examples of such hard particles include metal oxides, carbides, nitrides and the like. However, a problem with electro-deposition is that alloys based on aluminium metal are precluded from deposition from aqueous solutions and can only be deposited from fused-salt mixtures or solvent type solutions which renders aluminium-based overlay alloys impractical by this method of deposition. Further disadvantages are that such processes are generally expensive; even the best electro-deposited materials have marginal performance in the most demanding engine applications; and, the dimensional accuracy of the deposited overlay coating is somewhat lacking as coatings of this type are usually used in the as-deposited and unmachined condition.
In recent years people have attempted to improve the properties of overlays by depositing them by cathodic sputtering. This process enables overlay compositions based on an aluminium matrix to be deposited and also allows the deposition or generation of hard phases such as oxides and the like to be incorporated into the overlay alloy matrix. Cathodic sputtering is generally carried out at very high vacuums of about 10−6 torr which makes the process very expensive since only batch processes coating relatively small numbers of bearings at a time are possible and the sputtering process is inherently slow. DE 28 53 724 C describes the deposition of sliding coatings for bearings by cathodic sputter deposition. The coatings described include coatings based on aluminium alloys and are provided with a true dispersion of aluminium oxide formed in statu nascendi. Thus, the oxide content is generated by the oxidation of aluminium atoms as they are deposited due to the sputter chamber being provided with a source of oxygen. An example of a sputter deposited coating having a composition of Al20SnCu is given and which has a hardness of 130 Hv (which is harder than annealed mild steel, for example) compared with that of a cast material of the same composition which has a hardness of 35 Hv. The high hardness was maintained even after a heat treatment of 100 hours at 170° C. However, whilst such hard materials are likely to have improved wear resistance, they necessitate the use of a very hard co-operating shaft journal if excessive wear thereof is not to occur and they also have very inferior dirt embeddability properties. Indeed, the hardness and resistance to softening after heat treatment of such alloys made by cathodic sputtering is attributable to the aluminium and oxide phases being deposited on an atomic scale and producing a true dispersion hardened material according to the metallurgical definition thereof. As with electro-deposited coatings, the overlays produced are generally used in the as-deposited and unmachined condition and consequently dimensional accuracy is not as high as desired. Such dispersion hardened, sputter produced overlays may typically be harder by a factor of ×2 or more than the underlying bearing alloy on which they are deposited leading to unfavourable stress/strain distributions in operation. However, overlays produced by sputtering have produced the strongest overlay coatings currently available.
WO 99/47723, of common ownership herewith, describes the deposition of bearing alloy layers based on aluminium by high velocity oxy-fuel spraying (HVOF). The aluminium alloys containing tin or lead need to be heat treated after deposition in order to precipitate out and coarsen the soft phase in order to prevent excessive corrosion of the overlay coating layer under engine operating environments. Such heat treatments add to the cost of producing bearings and can have other undesirable effects. However, such spray-deposited coatings are generally machined after deposition thus, accuracy is high.